pebidiaist rocks oe pembrokeshire. 161 



Conclusions. 



We are now, I think, in a position to attempt an inquiry into 

 the probable physical conditions, locally at least, of these Pre- Cam- 

 brian epochs. 



In the oldest formation, the Dimetian, we have not, unfortunately, 

 at St. David's, a base to start from, since in no case probably have 

 we reached the lowest beds. There are no true conglomerates in 

 the whole series ; and of the lowest beds, or those which seem to 

 form the centre of an anticlinal, we are not sure whether they may 

 not even be intrusive. The next beds in this group are a great 

 series of altered quartz rocks and very fine-grained quartzites ; but 

 with these are associated a few of the ashy-looking bands mentioned 

 as either intrusive along the bedding or contemporaneous basaltic 

 flows. These beds alternate with one another, and hence appear as 

 if contemporaneous. The prevalence of volcanic action at this time 

 is therefore rendered more than probable. Still the enormous pre- 

 ponderance of quartz rocks throughout the whole formation, and the 

 absence of ashy beds and of felstone lavas and volcanic breccias of 

 any great importance, indicate to my mind that it was probably 

 chiefly detrital in its origin, the result of the wearing-down of pre- 

 viously existing rocks. "Whether this formation is chiefly made up 

 of detrital or volcanic materials, however, is a question chiefly for 

 the chemist and mineralogist. 



The question which to us is now of importance is that of its 

 position geologically ; and upon this point I venture to speak with 

 some confidence. The stratification shows undoubtedly that it is 

 older than, and that it is overlain unconformably by, the Pebidian 

 formation. Moreover, fragments of its several rocks are found in 

 the beds of the Pebidian which rest immediately upon it ; and hence 

 it must have been in existence, either in the form of an island or as 

 a portion of a continent, in pre-Pebidian times. It was through its 

 consolidated rocks also that volcanoes burst forth ; and upon them 

 also was spread out the material which was afterwards depressed 

 below and sorted and rolled by the encroaching Pebidian sea. 



The depression of the land during the Pebidian epoch was un- 

 doubtedly accompanied throughout by volcanic action, chiefly sub- 

 marine ; and though we do not know to what thickness this series may 

 have attained, nor, consequently, to what extent depression went 

 on, we know that the close of the epoch was accompained by the re- 

 elevation of the parts depressed, and by great changes over vast 

 areas. The history of the changes which took place at this time we 

 may partly read in the well-known Lower Cambrian rocks, the next 

 series in order of deposition*; for, when their base-line is seen, they 



xxi., Dr. Dawson, 'Acadian Geology,' and Prof. Nicholson, Quart. Journ. Geol. 

 Soc. vol. xxix. p. 21, the Huronian Epoch in Canada and New Brunswick must 

 also have been a period of yolcanic activity, and the rocks are to a great extent 

 similar in character to those of the Pebidian formation on this side. 



* See papers by the author, Quart. Journ. Geol. Soc. -vol. xxxi., and Geol. 

 Mag. April 1876. 



